Panels made of laminated composite materials rather than high-strength metal have many desirable characteristics, and are being used in increasing frequency for aircraft. However, high-strength solid metal rivets are not suitable for use with composite materials because forming an upset head on the tail of the rivet against a composite panel may damage the laminate. One technique for dealing with that problem is to employ tubular rivets because the tail of a tubular rivet can be bulbed or deformed more easily than the tail of a solid rivet, and thus less force is needed to upset the tail and less force is transmitted to the panel. A disadvantage of the tubular rivet is that it is generally more expensive than a solid rivet and it does not have shear strength as high as a solid rivet.
Another approach is to employ more ductile or malleable materials which will deform more readily, and hence are less likely to damage a composite panel. That approach, however, also involves a compromise with regard to shear strength, as well as the tensile strength of the deformed head needed to hold the panels together.
U.S. Pat. Nos. 3,426,641 and 3,747,467 provide systems which permit the upset forming of high strength, solid shank rivets in relatively soft aluminum sheets without adversely affecting the hole. This is accomplished by the use of a special forming collar. This collar, however, requires separate handling during installation, thereby increasing installation cost and complexity.
A further approach is the use of rivets made of high strength materials which have solid shanks and tubular ends. The ends can be upset without adversely affecting the structure in which the hole is formed, but if the tubular end is unsupported while it is being upset, internal buckling, eccentric head formation and cracking of the tubular end can result. In addition, because there is no internal support of the tubular upset end, it may not effectively resist the relatively high tensile separation loads on the joint.
U.S. Pat. No. 4,126,076 to Rosman discloses a fastener having a solid shank with a tubular tail, and a core pin positioned in the tubular tail to prevent inward deformation of the tail. This adds to the expense of the fastener, as well as introducing the need to retain the core pin within the rivet, both before and after installation. Also, if it is desired to use a particularly hard material, even deforming a tubular tail has its limitations in terms of ease of deformation against a soft panel. To prevent cracking of the upset tail, the Rosman patent finds it necessary that only tubular upsettable material extend beyond the face of the workpiece. That is, the solid shank end does not extend beyond the workpiece.
Because of varying design or tolerance thicknesses of panels being joined, it is usually desirable that a particular fastener be designed so that it has a grip range which will satisfactorily accommodate such variations. If the fastener does not have a satisfactory grip range, its application may be greatly limited. Various techniques have been employed for causing fasteners to buckle or deform in a desired manner or location consistent with the grip range. U.S. Pat. No. 2,030,167--Miller, for example, discloses varying the wall thickness of a buckling sleeve to affect the location of the buckle.
A need exists for a rivet having the shear strength of a solid rivet and a tail which can be readily upset so as to be usable with softer workpieces. Further, such a fastener must accommodate a reasonable workpiece thickness range. It is, of course, necessary that the rivet be highly reliable and economical to fabricate and install.